1. Field of the Disclosure
Embodiments disclosed herein relate generally to secondary cutting structures for use on drilling tool assemblies. More specifically, embodiments disclosed herein relate to secondary cutting structures having a plurality of cutting element rows disposed on cutter blocks. More specifically still, embodiments disclosed herein relate to secondary cutting structures having more than two cutting element rows disposed on cutter blocks.
2. Background Art
FIG. 1A shows one example of a conventional drilling system for drilling an earth formation. The drilling system includes a drilling rig 10 used to turn a drilling tool assembly 12 that extends downward into a well bore 14. The drilling tool assembly 12 includes a drilling string 16, and a bottomhole assembly (BHA) 18, which is attached to the distal end of the drill string 16. The “distal end” of the drill string is the end furthest from the drilling rig.
The drill string 16 includes several joints of drill pipe 16a connected end to end through tool joints 16b. The drill string 16 is used to transmit drilling fluid (through its hollow core) and to transmit rotational power from the drill rig 10 to the BHA 18. In some cases the drill string 16 further includes additional components such as subs, pup joints, etc.
The BHA 18 includes at least a drill bit 20. Typical BHA's may also include additional components attached between the drill string 16 and the drill bit 20. Examples of additional BHA components include drill collars, stabilizers, measurement-while-drilling (MWD) tools, logging-while-drilling (LWD) tools, subs, hole enlargement devices (e.g., hole openers and reamers), jars, accelerators, thrusters, downhole motors, and rotary steerable systems. In certain BHA designs, the BHA may include a drill bit 20 or at least one secondary cutting structure or both.
In general, drilling tool assemblies 12 may include other drilling components and accessories, such as special valves, kelly cocks, blowout preventers, and safety valves. Additional components included in a drilling tool assembly 12 may be considered a part of the drill string 16 or a part of the BHA 18 depending on their locations in the drilling tool assembly 12.
The drill bit 20 in the BHA 18 may be any type of drill bit suitable for drilling earth formation. Two common types of drill bits used for drilling earth formations are fixed-cutter (or fixed-head) bits and roller cone bits.
In the drilling of oil and gas wells, concentric casing strings are installed and cemented in the borehole as drilling progresses to increasing depths. Each new casing string is supported within the previously installed casing string, thereby limiting the annular area available for the cementing operation. Further, as successively smaller diameter casing strings are suspended, the flow area for the production of oil and gas is reduced. Therefore, to increase the annular space for the cementing operation, and to increase the production flow area, it is often desirable to enlarge the borehole below the terminal end of the previously cased borehole. By enlarging the borehole, a larger annular area is provided for subsequently installing and cementing a larger casing string than would have been possible otherwise. Accordingly, by enlarging the borehole below the previously cased borehole, the bottom of the formation can be reached with comparatively larger diameter casing, thereby providing more flow area for the production of oil and gas.
Various methods have been devised for passing a drilling assembly through an existing cased borehole and enlarging the borehole below the casing. One such method is the use of an underreamer, which has basically two operative states—a closed or collapsed state, where the diameter of the tool is sufficiently small to allow the tool to pass through the existing cased borehole, and an open or partly expanded state, where one or more arms with cutters on the ends thereof extend from the body of the tool. In this latter position, the underreamer enlarges the borehole diameter as the tool is rotated and lowered in the borehole.
A “drilling type” underreamer is typically used in conjunction with a conventional pilot drill bit positioned below or downstream of the underreamer. The pilot bit can drill the borehole at the same time as the underreamer enlarges the borehole formed by the bit. Underreamers of this type usually have hinged arms with roller cone cutters attached thereto. Most of the prior art underreamers use swing out cutter arms that are pivoted at an end opposite the cutting end of the cutting arms, and the cutter arms are actuated by mechanical or hydraulic forces acting on the arms to extend or retract them. Typical examples of these types of underreamers are found in U.S. Pat. Nos. 3,224,507; 3,425,500 and 4,055,226. In some designs, these pivoted arms tend to break during the drilling operation and must be removed or “fished” out of the borehole before the drilling operation can continue. The traditional underreamer tool typically has rotary cutter pocket recesses formed in the body for storing the retracted arms and roller cone cutters when the tool is in a closed state. The pocket recesses form large cavities in the underreamer body, which requires the removal of the structural metal forming the body, thereby compromising the strength and the hydraulic capacity of the underreamer. Accordingly, these prior art underreamers may not be capable of underreaming harder rock formations, or may have unacceptably slow rates of penetration, and they are not optimized for the high fluid flow rates required. The pocket recesses also tend to fill with debris from the drilling operation, which hinders collapsing of the arms. If the arms do not fully collapse, the drill string may easily hang up in the borehole when an attempt is made to remove the string from the borehole.
Recently, expandable underreamers having arms with blades that carry cutting elements have found increased use. Expandable underreamers allow a drilling operator to run the underreamer to a desired depth within a borehole, actuate the underreamer from a collapsed position to an expanded position, and enlarge a borehole to a desired diameter. Cutting elements of expandable underreamers may allow for underreaming, stabilizing, or backreaming, depending on the position and orientation of the cutting elements on the blades. Such underreaming may thereby enlarge a borehole by 15-40%, or greater, depending on the application and the specific underreamer design.
Typically, expandable underreamer design includes placing two blades in groups, referred to as a block, around a tubular body of the tool. A first blade, referred to as a leading blade absorbs a majority of the load, the leading load, as the tool contacts formation. A second blade, referred to as a trailing blade, and positioned rotationally behind the leading blade on the tubular body then absorbs a trailing load, which is less than the leading load. Thus, the cutting elements of the leading blade traditionally bear a majority of the load, while cutting elements of the trailing blade only absorb a majority of the load after failure of the cutting elements of the leading blade. Such design principles, resulting in unbalanced load conditions on adjacent blades, often result in premature failure of cutting elements, blades, and subsequently, the underreamer.
Accordingly, there exists a need for apparatuses and methods of designing secondary cutting structures having unique cutting element, blade, and block design.